Cutaneous mosaic disorders are severe, rare genetic skin disorders appearing in patterns due to somatic mutation during embryonic development. The timing of mutation determines the identity and extent of affected tissue, and given contributions of ectoderm (keratinocytes), mesoderm (fibroblasts, cutaneous vessels), and neural crest (melanocytes) to the skin, one or multiple cell types can be affected. We have found that consequences of such mutations can be severe as in rapidly-growing, treatment-unresponsive congenital hemangiomas due to GNA14 mutation, or severe osteomalacia in the cutaneous-skeletal hypophosphatemia syndrome due to multi-lineage somatic activating RAS mutations. Next generation sequencing has powered gene discovery in cutaneous mosaic disorders, and we have successfully identified several novel genetic causes of these conditions. We now propose to expand our cohort of well-phenotyped cutaneous mosaic disorders, to screen for mutations in potential causative genes, and to employ exome sequencing in mutation-unknown subjects with reflex to genome sequencing for unsolved cases to discover novel genetic causes. Included in this cohort are proliferative/hamartomatous conditions including congenital fascial dystrophy, nevus comedonicus, and congenital hemangiomas, as well as rare mosaic presentations of common disorders including acne, psoriasis, lichen planus, and discoid lupus erythematosus, which provide an opportunity to identify pathways relevant to autoimmunity and inflammation. We will utilize patient-derived cells and tissue to interrogate the function of identified novel genes, and will employ transgenic tissue equivalents to study and prove pathogenesis of identified mutations when possible. For a limited number of compelling, novel genes previously unrecognized to be relevant to cutaneous biology, we will employ mouse models including knockout and CRISPR-generated knockin lines to prove pathogenesis of identified mutations and to provide initial insights into disease pathobiology. These studies will continue to identify molecular pathways central to the complex processes of epidermal differentiation, self-renewal, and inflammation, and will provide critical context for future biologic studies and development of novel therapeutics.